A barometric-based approach for estimating an altitude of a mobile device requires (i) an accurate estimate of pressure at a location of the mobile device, (ii) an accurate estimate of pressure at a location of a remote pressure sensor, (iii) an accurate estimate of temperature at a location of a remote temperature sensor or at the location of the mobile device, and (iv) a known altitude of the remote pressure sensor, where the location of the remote pressure sensor and the location of the remote temperature sensor may be the same location or different locations. An estimate of an altitude of the mobile device (hmobile) can be computed as follows:
                                          h            mobile                    =                                    h              sensor                        +                                                            RT                  remote                                gM                            ⁢                              ln                ⁡                                  (                                                            P                      sensor                                                              P                      mobile                                                        )                                                                    ,                            (                  Equation          ⁢                                          ⁢          1                )            where Pmobile is the estimate of pressure at the location of the mobile device, Psensor is the accurate estimate of pressure at the location of the remote pressure sensor, Tremote is the accurate estimate of temperature at the location of the remote temperature sensor, hsensor is the known altitude of the remote pressure sensor, g corresponds to the acceleration due to gravity, R is the universal gas constant, and M is the molar mass of air (e.g., dry air or other).
In some cases, the estimate of pressure at the location of the remote pressure sensor is converted to a reference pressure that specifies an estimate of pressure at a reference altitude as follows:
                                          P            ref                    =                                    P              sensor                        ⁢                          exp              ⁡                              (                                  -                                                            gM                      ⁡                                              (                                                                              h                            ref                                                    -                                                      h                            sensor                                                                          )                                                                                    RT                      remote                                                                      )                                                    ,                            (                  Equation          ⁢                                          ⁢          2                )            where Psensor is the accurate estimate of pressure at the location of the remote pressure sensor, Pref is the reference pressure at a reference altitude, and href is the reference altitude. The altitude of the mobile device, hmobile is computed using Equation 1, where href is substituted for hsensor and Pref is substituted for Psensor. The reference altitude, href may be any altitude, and is often set at sea-level.
When two or more reference pressures are available, the reference pressures are combined into a single reference pressure value (e.g., using an average, weighted average, or other suitable combination of the reference pressures), and the single reference pressure value is used for the reference pressure, Pref.
As one of ordinary skill in the art will appreciate from Equation 1 and Equation 2, the estimate of the mobile device's altitude, hmobile, will be not be accurate if the estimate of the remote temperature, Tremote, is not accurate. Accuracy of the estimate of the remote temperature, Tremote, is particularly critical since it impacts the accuracy of the reference pressure, Pref from Equation 2, and the accuracy of the estimate of the mobile device's altitude, hmobile, from Equation 1.
Unfortunately, a reference pressure can be computed using a temperature measurement of unknown accuracy, as is often the case for many crowdsourced weather stations like NOAA, Weather Underground, Earth Networks, or hobby grade weather stations installed by private individuals at their homes or businesses. In some cases, the temperature measurement is not accurate, which makes the reference pressure unreliable for use in computing an estimate of an altitude of a mobile device. Described below are systems and methods for determining if a reference pressure based on an estimate of a remote temperature is accurate enough for use in computing an estimate of a mobile device's altitude, and for determining other data for use in computing an estimate of a mobile device's altitude when the reference pressure is determined to not be accurate enough for use in computing an estimate of a mobile device's altitude.